Stefano Tirelli

Fully Passivated AlInN/GaN HEMTs With

We report the fabrication and characterization of 30-nm-gate fully passivated AlInN/GaN high-electron mobility transistors (HEMTs) with cutoff frequencies <i>f</i>_T and <i>f</i>_MAX simultaneously exceeding 200 GHz at a given bias point. The current gain cutoff frequency does not vary significantly for 2.5 <; <i>V</i>_DS <; 10 V, while <i>f</i>_MAX reaches a maximum value of <i>f</i>_MAX = 230 GHz at <i>V</i>_DS = 6 V. This is the first realization of fully passivated AlInN/GaN HEMTs with <i>f</i>_T/<i>f</i>_MAX ≥ 205 GHz, a performance enabled by the careful shaping of the gate electrode profile and the use of a thin 60-nm SiN encapsulation film.

f_{\rm T}/f_{\rm MAX}

We report a new generation of high-performance AlGaN/GaN high-electron-mobility transistors (HEMTs) grown on high-resistivity Si (111) substrates. We map out small- and large-signal device performances against technological parameters such as the gate length and the source-drain contact separation. We report the first large-signal performance for a GaN-on-Si technology offering an output power of 2 W/mm and an associated peak power-added efficiency of 13.8% (peak of 18.5%) at 40 GHz without any field plate. The technology offers measured transconductances of up to 540 mS/mm and cutoff frequencies as high as fT/fMAX = 152/149 GHz at a given bias point. These are the highest cutoff frequencies to date for fully passivated AlGaN/GaN HEMTs on silicon substrates. The results confirm GaN-on-Si technology as a promising contender for low-cost millimeter-wave power electronic applications.

of 205/220 GHz

We report high-speed fully passivated deep submicrometer (Al,Ga)N/GaN high-electron mobility transistors (HEMTs) grown on (111) high-resistivity silicon with current gain cutoff frequencies of as high as fT = 107 GHz and maximum oscillation frequencies reaching fMAX = 150 GHz. Together, these are the highest fT and fMAX values achieved for GaN-based HEMTs implemented on silicon substrates to date. The performance reported here is competitive with recently published results for similar geometry high-performance passivated HEMTs on semi-insulating silicon-carbide substrates.

150-GHz Cutoff Frequencies and 2-W/mm Output Power at 40 GHz in a Millimeter-Wave AlGaN/GaN HEMT Technology on Silicon

We report the first 94-GHz (W-band) large-signal performance of AlInN/GaN high-electron-mobility transistors (HEMTs) grown on high-resistivity silicon (111) substrates. A maximum output power density of 1.35 W/mm and peak power-added-efficiency of 12% are measured at 94 GHz. The devices exhibit a dc maximum current drain density of 1.6 A/mm and a peak transconductance of 650 mS/mm. In small-signal operation, cutoff frequencies fT/fMAX = 141/232 GHz are achieved. The large-signal performance of our AlInN/GaN HEMTs on silicon at 94 GHz stills lags the best reported results one on SiC substrates but nevertheless confirms the tremendous interest of GaN-on-Si HEMT technology for low-cost millimeterwave electronic applications.

107-GHz (Al,Ga)N/GaN HEMTs on Silicon With Improved Maximum Oscillation Frequencies

We report the large-signal performance of high electron mobility transistors (HEMTs) fabricated on GaN- and AlN-capped AlInN/GaN epilayers grown on semi-insulating SiC substrates. Large-signal measurements at 10 and 40 GHz are presented with both gate and drain dynamic loadlines to clarify the factors limiting the high-power performance. Devices fabricated with AlN-capped epilayers show a marginal advantage in terms of higher current and reduced dispersion, but GaN-capped epilayers perform better in terms of reduced short-channel effects and better channel control. In large-signal operation at 40 GHz, both device types delivered power densities in excess of 4.5 W/mm. A maximum power density of 5.8 W/mm is achieved on GaN-capped devices which is, to the best of our knowledge, the highest power density reported at 40 GHz in AlInN/GaN-based HEMTs.

94-GHz Large-Signal Operation of AlInN/GaN High-Electron-Mobility Transistors on Silicon With Regrown Ohmic Contacts

A new W-band active load-pull system is presented. It is the first load-pull system to implement a 94 GHz load by means of an active loop exploiting frequency conversion techniques. The active loop configuration demonstrates a number of advantages that overcome the typical limitations of W-band passive tuners or conventional active open-loop techniques in a cost-effective way: load reflection coefficients ΓL as high as 0.95 in magnitude can be achieved at 94 GHz, thus providing a nearly full coverage of the Smith chart. Possible applications of the setup include technology assessment, large-signal device model verification at sub-terahertz frequencies, and W-band monolithic microwave integrated circuit design and characterization. The availability of direct and accurate load-pull measurements at W-band should prove an asset in the development of sub-terahertz integrated circuits. First measurements performed on high-performance InP double heterojunction bipolar transistors and GaN high electron-mobility transistors are presented.

AlInN-Based HEMTs for Large-Signal Operation at 40 GHz

AlGaN/GaN 70-nm-gate high-electron mobility transistors (HEMTs) fabricated using either ion implantation or conventional mesa isolation are compared. Although the resulting devices display comparable dc characteristics, the isolation process influences the RF and pulsed I-V characteristics. Although others have described implant-isolated GaN HEMTs, published reports focused on limited performance metrics, such as the gate leakage current. The present multiparametric study explicitly contrasts the performance of ion-implanted devices to otherwise identical mesa-isolated deep-submicrometer high-speed AlGaN/GaN HEMTs, in terms of transistor cutoff frequencies, small-signal model parameters, microwave noise performance, gate leakage currents, and large-signal pulsed I-V characteristics. We find that implant isolation can bring compelling advantages in terms of bandwidth, microwave noise performance, and tighter parametric distributions.

A

We report on the low-temperature growth of heavily Si-doped (>1020 cm−3) n+-type GaN by N-rich ammonia molecular beam epitaxy (MBE) with very low bulk resistivity (<4 × 10−4 Ω·cm). This is applied to the realization of regrown ohmic contacts on InAlN/GaN high electron mobility transistors. A low n+-GaN/2 dimensional electron gas contact resistivity of 0.11 Ω·mm is measured, provided an optimized surface preparation procedure, which is shown to be critical. This proves the great potentials of ammonia MBE for the realization of high performance electronic devices.

W

We present a complete methodology to evaluate the accuracy of microwave transistor figures-of-merit fT (current gain cut-off frequency) and fMAX (maximum oscillation frequency). These figures-of-merit are usually extracted from calibrated S-parameter measurements affected by residual calibration and measurement uncertainties. Thus, the uncertainties associated to fT and fMAX can be evaluated only after an accurate computation of the S-parameters uncertainties, including the contribution from de-embedding. This was done with the aid of two recently released software tools. We also present an analysis on how different interpolation/extrapolation methodologies affect uncertainty. Finally, an overview of the possible causes of errors and suggestions on how to avoid them are given. With the continued rise of reported fT /fMAX values, this study has become necessary in order to add confidence intervals to these figures-of-merit

-Band On-Wafer Active Load–Pull System Based on Down-Conversion Techniques

We report the characterization of GaN high electron mobility transistors (HEMTs) using a new AlN-capped AlInN/GaN epilayer structure developed to achieve high current densities and reduced gate leakage currents. Devices with gate lengths of 75 and 200 nm and various and source-drain separations were fabricated simultaneously, allowing the selection of the most favorable configuration for power performance. We show that, as anticipated, aggressive scaling of source-drain spacing and gatelength does not benefit power performance because of early breakdown and more pronounced short-channel effects. For a non-field-plated 0.2 mu m gate length in a 4 mu m source-drain gap, the new epitaxial structure achieved a power density of 4.5W/mm at 40 GHz. To the best of our knowledge, this is the highest power reported at 40 GHz for AlInN/GaN-based transistors, and the first report of the large-signal performance of an AlN-capped AlInN/GaN-based HEMT. (c) 2013 The Japan Society of Applied Physics